1. Field of the Invention
The present invention generally relates to a light emitting diode (LED) array head. The invention relates, more particularly, to an LED array head including focusing lenses for use in an optical writing system of an optical printer, a digital copying machine, a facsimile machine or other printing device.
2. Discussion of the Background
LED elements are more reliable for use under vibrations and noises than laser beam printers (LBP). They are also suitable for miniaturizing an optical head since an apparatus with an LED array head does not need a scanning mechanism such as a polygonal mirror as needed by an LBP using a semiconductor laser.
On the other hand, the LED elements have problems. For example, the quality of light from each element is dispersive and beam spot shapes are not uniform. The dispersed quality of light and the lack of uniformity of the beam spot shapes generate an uneven dot density of an output image such as an image containing black stripes. This unevenness becomes more remarkable in multi-valued images such as picture images. These problems generally originate from a unit magnification focusing element of a rod lens array.
FIG. 10 illustrates a conventional rod lens array which is made up of a plurality of rod lenses 104. These lenses may be made of fiber optic elements 104 which may be unit magnification focusing elements. The rod lenses 104 may be constructed as illustrated in FIG. 11 in which a fiber optic element 104 is illustrated as having a cladding 110 and a core 112. In FIG. 11, it is illustrated that the light reflects between the boundary of the cladding and the core. An alternative embodiment of the rod lens 104 is illustrated in FIG. 12 which shows a gradient index lens. In this Figure, it is seen that the light does not sharply reflect within the fiber optic lens 104 but the direction of the light is smoothly changed as illustrated in FIG. 12.
The disadvantages of the rod lens arrays including arrays which use gradient index lenses are caused by two problems. First, a dispersed light loss may occur at connecting portions between the rod lenses. This light loss occurs near the connecting portions due to a difference in a p itch of emitting elements and that of focusing elements. Second, it is difficult to uniformly converge light beams since flared light such as reflected light in a rod lens easily occurs.
FIG. 13 illustrates a conventional roof mirror lens array. In this system, light originating at 120, is reflected off of a mirror 122 through a lens 124, off of a roof mirror array 126, back through the lens 124, and off of a lower surface of the mirror 122 in order to form the image 128. This focusing system also has the above two problems.